The present invention pertains to medical nuclear magnetic resonance scanners, and, more particularly, to the use of vibration isolators in connection therewith to prevent degradation of scanner images due to vibration of the magnet structure caused by external influences.
Since the invention of the medical nuclear magnetic resonance (NMR) scanning technique by Dr. Raymond Damadian, as set forth in U.S. Pat. No. 3,789,832, this technique has been widely adopted in the medical arts. Medical NMR scanning requires creation of a substantial constant "primary" magnetic field passing through the patient's body. Additional "gradient" magnetic fields varying with time typically are superimposed on the primary field. The patient is exposed to radio frequency electromagnetic waves which also vary with time in particular patterns. Under the influence of the magnetic fields and the radio waves, certain atomic nuclei within the patient's tissues resonate and emit further radio waves. By mathematical techniques involving correlation of the magnetic field patterns in use at various times with the radio frequency waves emitted, it is possible to determine the amounts and physical states of particular atomic nuclei at various locations within the patient's body. This information typically is displayed as an image with shadings corresponding to the concentration and/or physical state of certain chemical substances of interest. These parameters ordinarily differ for differing kinds of tissues. Thus, the image created by NMR techniques permits the physician to see organs and soft tissues within the body, and also permits the physician to see abnormalities, such as tumors, within the body. Accordingly, NMR scanning and imaging techniques are being adopted rapidly by physicians.
Medical NMR scanning imposes certain challenging technical requirements for the apparatus. The primary magnetic field must be a strong field, typically on the order of about 1 kilogauss or more and often more than about 10 kilogauss (1 Tesla), far stronger than the magnetic fields associated with common magnets. Moreover, the primary magnetic field must be precisely configured. Thus, the primary field, before application of the gradient fields, should be uniform and constant to at least about 1 part in 1,000, and preferably at least about 1 part in 10,000 or better, in order to provide a useful image. Even better uniformity is more desirable. This uniform and constant primary magnetic field must be maintained over a region of the patient's body large enough to provide medically useful information, typically over the scanning region encompassing a major portion of the cross section through the patient's torso. Further, the magnetic field apparatus typically must be arranged to receive the patient's body, and hence must provide openings large enough for the patient's body to fit within the apparatus. All these requirements, taken together, pose a formidable technical problem.
Two distinct and fundamentally different approaches to these requirements are currently employed in construction of medical NMR scanners. As set forth in commonly assigned U.S. Pat. No. 4,675,609 to Danby et al., magnetic field producing means such as permanent magnets can be combined with a ferromagnetic metal frame and other components to form a magnetic assembly which provides the primary field. The disclosure of said U.S. Pat. No. 4,675,609 is hereby incorporated by reference herein. Medical NMR scanners incorporating magnetic assemblies according to U.S. Pat. No. 4,675,609 have excellent primary fields and hence offer good scanning capabilities.
Additional configurations of primary field magnets employing electromagnetic coils as flux-producing elements in conjunction with ferromagnetic frames are disclosed in commonly assigned U.S. Pat. No. 4,766,378 to Danby et al., the disclosure of which is hereby incorporated by reference herein. These coils may be formed from superconducting materials to maximize the current-carrying capacity of the coils and thereby maximize the field strength. Other primary field magnets utilizing superconducting coils without ferromagnetic frames have also been employed. The primary field magnet structures utilized in typical NMR scanners are massive devices. Thus, the ferromagnetic frames and permanent magnets according to either of the aforementioned patents typically weigh many tons. These massive magnet assemblies ordinarily are mounted on the floor of a building. Alternatively, the magnet assembly may be secured to the frame of a heavy truck which can be parked at a hospital or other institution where the scanner is to be operated.
Modern NMR scanners have been developed to the point where they provide images of extraordinary quality. It has long been recognized that movement of the patient relative to the scanning equipment during the procedure can degrade the image. It has further been recognized that external magnetic fields and magnetic field disturbances caused by ferromagnetic materials in the vicinity of the scanner can substantially degrade the image quality. Considerable efforts have been devoted heretofore towards alleviating these effects. Thus, particular sequences of radio frequency excitation signals and magnetic field gradient have been developed which substantially compensate for motion of the patient. Also, primary field magnets which resist external influences to a substantial degree have been developed. For example, the magnet assemblies according to the aforementioned '609 and '378 patents effectively isolate the patient receiving space from any external field influences by virture of the ferromagnetic frames incorporated in such assemblies.
Despite all of the improvements made to NMR scanning apparatus and techniques heretofore, however, there has still been a need for further improvements. It has been found that an NMR scanning apparatus installed at one location does not provide the same, performance as equivalent apparatus installed at another location. Thus, two scanning units of identical design installed at two different locations may produce images of different quality. A scanner at one location may provide perfectly good images, whereas the images provided by an ostensibly identical scanner installed at another location may be of significantly lower quality. These quality variations have persisted to some extent despite intensive efforts to identify and eliminate known causes of image degradation such as patient movement and external magnetic field disturbances. Thus, prior to the present invention it has been the practice to simply accept the somewhat poorer images produced by some machines at some locations. Accordingly, there has been a need heretofore for further improvements in NMR scanners which would eliminate these unexplained variations and bring all the scanners at all locations up to the image quality standards heretofore provided by only those machines at the best locations.
One aspect of the present invention incorporates the discovery that these unexplained variations in image quality are due at least in part to mechanical vibrations transmitted to the scanning apparatus through the base structure on which the scanner is mounted. Thus, according to the present invention it has been found that where an NMR medical scanning apparatus is installed in a building or other environment subject to vibration either of a continuous nature (e.g., from rotating machinery such as components of air conditioning equipment located in nearby spaces) or of a transitory nature (e.g., from shocks, impulses or vibrations caused by trucks or trains passing the vicinity of the building), the image degrades, apparently due to vibration of the magnet structure. The relationship between such image degradation and external vibration sources had not been associated heretofore as the magnet structures typically weigh many tons, and hence would not be able to vibrate to any substantial degree. Accordingly, the discovery of such relationship was quite surprising and unexpected.
Although the present invention is not limited by any theory of operation, it is believed that vibration of the magnet structure may cause relative movement of ferromagnetic elements in the magnetic flux path. For example, in a permanent magnet structure incorporating a plurality of magnetic blocks, vibration of the magnet structure may cause minute movements of these blocks. Likewise, in a magnet structure incorporating a magnetically permeable, ferromagnetic frame, vibrations may cause minute movements of frame elements relative to one another and may also cause movement of the magnetic flux producing elements relative to the frame. Thus, the permanent magnets or electromagnets used with such a frame may move slightly relative to the frame. Any of these various types of relative movement may cause subtle changes in the magnetic flux path and hence may cause changes in the magnetic field applied to the patient by the primary field magnet. Such changes occurring during the scanning cycle would disrupt the imaging process and degrade the image. Regardless of the theory of operation, it has been found, according to the present invention, that the performance of NMR scanning apparatus can be materially enhanced by isolating the magnetic field producing elements, and desirably the frame as well, from externally applied transitory and continuous vibrations. The performance enhancements provided by such isolation are particularly significant where the magnetic field producing elements incorporate a plurality of permanent magnets and where the frame incorporates a plurality of juxtaposed magnetic elements providing a flux path and carrying magnetic flux to or from the patient receiving space.
It is, thus, apparent that there has heretofore been a significant but unrecognized need to provide medical NMR scanners in which image degradation is overcome by minimizing the introduction of external vibrations into the interior of the scanners. The present invention provides NMR scanners which meet that need.